Compositions of organopolysiloxane and hydrophobic oxidic reinforcing filler can be prepared by two basic processes:
In the first process, hydrophilic filler and organopolysiloxane are mixed in the presence of a hydrophobizing agent for the filler. However, the process has the disadvantage of a long batch running time and high emissions, which occur at many points and are difficult to control. Specific control of the hydrophobizing is scarcely possible, and corrections to the filler contents of the compositions are no longer possible, since suitable fillers are lacking.
In the second process, prehydrophobized filler and organopolysiloxane are mixed. A process for hydrophobizing oxidic reinforcing filler is described in U.S. Pat. No. 5,057,151 where the hydrophilic filler is subjected to mechanical stress in an excess of hydrophobizing agent. Excess hydrophobizing agent is then removed and recycled to the process.
During prehydrophobizing of the filler, it is possible to control the degree of hydrophobizing specifically and to vary it within wide limits, high and/or uniform degrees of hydrophobizing being possible, which is a prerequisite for many uses. The prehydrophobizing process allows the filler to be hydrophobized such that crosslinkable organopolysiloxane compositions can be prepared by simple mixing of the hydrophobized filler with organopolysiloxane. The use of previously hydrophobized filler leads to a significant increase in the capacity of the mixing units. Emissions are limited to a central unit and as a result are easier to control. The consumption of hydrophobizing agent can be reduced significantly compared with the in situ process. The filler content of the compositions can easily be corrected by addition of further filler.
Organopolysiloxane compositions which comprise a prehydrophobized reinforcing filler, such as pyrogenically prepared silicic acid, show an increase in viscosity during storage. For example, the increase in viscosity can be so severe that after a storage time of a few weeks the mass shows rubber-elastic properties and therefore is no longer processible. In the case of peroxidically crosslinking organopolysiloxane compositions, the increase in viscosity can be compensated for by addition of a low-viscosity organopolysiloxane. This is not possible in the case of organopolysiloxane compositions which crosslink by addition.